Heat flux measuring system

ABSTRACT

A heat flux meter having thermocouples responsive to temperature differentials between two similar surfaces, the heat flux to one of which is to be determined, a first heater responsive to the thermocouples for heating the other surface, a second heater responsive to temperature differences between it and the first heater to prevent heat flow away from the other surface and a reflective shield for preventing environmental heat flow to the other surface.

nited States Patent 1 Adams et al.

[ HEAT FLUX MEASURING SYSTEM [75] Inventors: Donald E. Adams,Williamsville; Franklin A. Vassallo, Lancaster, both of NY.

[73] Assignee: Cornell Aeronautical Laboratory, lnc., Buffalo, NY.

[22] Filed: Nov. 3, 1970 [21] App1.No.: 86,552

[52] US. Cl. ..73/l90 H [51] Int. Cl. ..G0lk 17/00 [58] Field of Search..73/15, 190, 340 HF [56] References Cited UNITED STATES PATENTS3,217,538 11/1965 Loeb ..73/190 3,256,734 6/1966 Storke ..73/l93 13,720,103 1March 13, 1973 3,367,182 2/1968 Baxter ..73/190 3,267,7288/1966 Solomons ..73/19O FOREIGN PATENTS OR APPLICATIONS 442,294 3/1927Germany ..73/190 Primary ExaminerRichard C. Queisser AssistantExaminer-Herbert Goldstein Attorney-Allen J. Jaffe [57] ABSTRACT A heatflux meter having thermocouples responsive to temperature differentialsbetween two similar surfaces, the heat flux to one of which is to bedetermined, a first heater responsive to the thermocouples for heatingthe other surface, a second heater responsive to temperature differencesbetween it and the first heater to prevent heat flow away from the othersurface and a reflective shield for preventingenvironmental heat flow tothe other surface.

5 Claims, 1 Drawing Figure PATENTEUHARI 3l973 ATTORNEY HEAT FLUXMEASURING SYSTEM BACKGROUND OF THE INVENTION The present inventionrelates to a heat flux measuring system, and, more specifically, to aheat flux sensor for measuring the thermal flux from a combinedradiation and convection heat source to any surface independent of theemissivity or thermal characteristics thereof.

Prior to flight tests of advanced aerodynamic structures, structuraltests under simulated flight conditions are generally necessary.Laboratory tests of simulated aerodynamic heating may be conducted byheating the test structure with a radiation source that provides thesame rate of heating as calculated for actual flight.

Presently, thermal flux sensors are known for controlling the thermalenvironment, but these are dependent upon a knowledge of the teststructures thermal properties. One type of sensor bases its operation ona temperature drop across a material of known thermal properties whereasa second type bases its operation on the rate of temperature rise in amaterial of known heat capacity. These techniques require that thethermal properties and surface emissivity of the heated material beknown. The surface temperature of the sensor may also be at a differenttemperature than the surface to be measured, resulting in errors inthermal flux measurement.

A more unusual thermal sensor is the isothermal heat meter. This sensorconsists of two resistive heating elements located at opposite ends of arod or composite of rods. One end of the rod is well insulated whereasthe other is exposed to the heat load to be measured. In operation, theexposed end is maintained at the vehicle skin temperature by applying aregulated amount of power to its surface heater. At the same time, theunexposed end is also maintained at the skin temperature using a similarheater. Under this symmetrical heating pattern, the net heat inflow issimply the difference in electrical energy applied to the rod ends.Because the sensor is always maintained equal in temperature to that ofthe skin, the net heat inflow to the sensor is also that to the skin.However, for radiation heating, the surface of the meter must have thesame emissivity as the structure surface. If this emissivity is notknown or if it varies during a heating cycle and cannot be made the sameon the meter as on the structure, the meter will not be capable ofmeasuring the thermal flux during radiation heating.

SUMMARY OF THE INVENTION The foregoing disadvantages of prior devicesare overcome according to the principles of the present invention whichprovides a thermal fiux meter for measuring the thermal flux from a heatsource to any surface independent of the emissivity or thermalproperties of that surface.

The thermal flux meter according to the present invention is based onthe principle that a surface temperature depends only on the net surfaceheat flux and the internal characteristics of the surface and not on themanner in which the heat is introduced. Thus, if one first surface isheated to maintain its temperature equal to that of a second adjacent orremote, similar surface, the heat input to which is to be determined,and the heat from the heat source for the first surface is absorbed onlyby the first surface then the heat flux to the first surface is equal tothe unknown heat flux to the second surface.

In accordance with the present invention a heater supplies heat to thefirst surface in accordance with the temperature differential betweenthe first and second surface. To insure that all the heat absorbed bythe first surface is due to the heater and that all the heat from theheater goes to the first surface, a highly reflective shield is providedbetween the environment and the first surface and a second heater isprovided between the shield and the first heater which eliminates anyheat flux between the two heaters.

Basically then, the present invention comprises; first thermal sensingmeans for generating a first signal responsive to the heat input to afirst surface; second thermal sensing means for generating a secondsignal responsive to the heat input to a second surface which is similarto the first surface; first heating means for applying heat to the firstsurface in response to the first and second signals; and means forshielding the first surface from the environment whereby the heat inputto the first surface is equal to the heat input to the second surface.

BRIEF DESCRIPTION OF THE DRAWING For a fuller understanding of thepresent invention reference should now be had to the followingdescription of the same taken in conjunction with the accompanyingdrawing wherein the only FIGURE is a schematic sectional view of theflux meter with the circuitry shown in block form.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to the drawing thethermal flux meter is generally depicted by the numeral 10 and comprisesa first heater 12, which may have a resistive heating element l4embedded therein. Heater I2 is bonded or otherwise suitably secured to asurface S, of a test vehicle or the like by means of a thin bondingagent 16 of high thermal conductance.

A second heater 18, which may have a resistive heating element 20embedded therein, is bonded or secured to the first heater 12 by meansof a securing or bonding element 22 of low thermal conductivity such asaluminum oxide. A rectangular, highly reflective shield 24 is providedas an enclosure for surface S, and heaters 12 and 18. Shield 24 may befabricated of stainless steel having a gold coating.

A first thermal sensor, such as thermocouple T,, is provided at surfaceS,. A second thermal sensor, such as thermocouple T is provided at asurface S the heat flux to which is to be measured. Surface S hasinternal characteristics similar to those of surface S, and may belocated adjacent to S, or at a distance remote therefrom. Second andthird thermal sensors in the form of thermocouples T and T, are locatedin heaters 12 and 18 to respond to the heat differential between theheaters.

The manner in which the heat flux meter operates to determine the heatflow to surface S will now be described. Thermocouples T, and T generatesignals which are indicative of the temperatures at surfaces S, and 8,,respectively. The difference between these signals is amplified bysuitable amplifier 26, the output from which is fed into a power source28 for actuating heating element 14. Heating element 14 will thusfunction to maintain the temperature T at surface S, equal to thetemperature T at surface S Since surface S is similar to surface s; andhas the same internal characteristics, the power generated at 28 will bea function of the unknown heat flux to S, when T equals T, only ifsubstantially all of the heat generated by heater 12 is delivered tosurface S,. It is also essential that substantially all the heatabsorbed at surface S is due to that delivered only by heater 12. Toinsure that all the heat from heater 12 flows to surface S heater 18 isactuated by power source 32 and amplifier 30 to maintain T equal T WhenT equals T no heat can flow from heater l2 upward toward heater 18. Toinsure that all of the heat delivered to surface S is due to heater 12and not from the environment, shield 24 functions to reflect the heatfrom the environment away from the internal parts of the meter andsurface 8,. A suitable indicator 34 can be provided to indicate thepower output of 28 in terms of the heat flux to surface S Although apreferred embodiment of the present invention has been described,changes will obviously occur to those skilled in the art. It istherefore intended that the invention is to be limited only by the scopeof the appended claims.

We claim:

1. A thermal flux meter, comprising;

a. first temperature sensing means for generating a first signalresponsive to the heat input to a first surface,

b. second temperature sensing means for generating a second signalresponsive to the heat input to a second surface which is similar tosaid first surface,

0. first heating means for applying heat to said first surface inresponse to the difference between said first and second signals,

d. means for preventing heat flow to said first surface from theenvironment and for preventing heat flow from said first heating meansto the environment including second heating means responsive to thetemperature difference between said first heating means, and the secondheating means and further including reflective means for shielding saidfirst surface from the environment,

e. said last mentioned means is spaced from said second surface wherebyheat flow can occur directly between the environment and said secondsurface.

2. The meter according to claim 1 wherein;

f. said first heating means is secured to said first sur face by anelement of high thermal conductance, and

g. said second heating means is secured to said first heating means by asecond element of low thermal conductivity.

3. The meter according to claim 2, wherein;

b. said first and second heating means include resistive heatingelements, and

i. said temperature sensors comprise thermocouples.

4. The meter according to claim 1, further comprising;

f. means for controlling the heat output of said second heating meanscomprising third and fourth thermal sensin means repponsive to temerature differential be ween said 1rst and secon heating means. 5. Themeter according to claim 4, wherein; g. said first and second heatingmeans include resistive heating elements, and h. said temperaturesensors comprise thermocouples.

1. A thermal flux meter, comprising; a. first temperature sensing meansfor generating a first signal responsive to the heat input to a firstsurface, b. second temperature sensing means for generating a secondsignal responsive to the heat input to a second surface which is similarto said first surface, c. first heating means for applying heat to saidfirst surface in response to the difference between said first andsecond signals, d. means for preventing heat flow to said first surfacefrom the environment and for preventing heat flow from said firstheating means to the environment including second heating meansresponsive to the temperature difference between said first heatingmeans, and the second heating means and further including reflectivemeans for shielding said first surface from the environment, e. saidlast mentioned means is spaced from said second surface whereby heatflow can occur directly between the environment and said secondsurface.
 1. A thermal flux meter, comprising; a. first temperaturesensing means for generating a first signal responsive to the heat inputto a first surface, b. second temperature sensing means for generating asecond signal responsive to the heat input to a second surface which issimilar to said first surface, c. first heating means for applying heatto said first surface in response to the difference between said firstand second signals, d. means for preventing heat flow to said firstsurface from the environment and for preventing heat flow from saidfirst heating means to the environment including second heating meansresponsive to the temperature difference between said first heatingmeans, and the second heating means and further including reflectivemeans for shielding said first surface from the environment, e. saidlast mentioned means is spaced from said second surface whereby heatflow can occur directly between the environment and said second surface.2. The meter according to claim 1 wherein; f. said first heating meansis secured to said first surface by an element of high thermalconductance, and g. said second heating means is secured to said firstheating means by a second element of low thermal conductivity.
 3. Themeter according to claim 2, wherein; h. said first and second heatingmeans include resistive heating elements, and i. said temperaturesensors comprise thermocouples.
 4. The meter according to claim 1,further comprising; f. means for controlling the heat output of saidsecond heating means comprising third and fourth thermal sensing meansresponsive to temperature differential between said first and secondheating means.